A DVD player to play back contents data recorded in an optical disk, such as a DVD-R, has been developed. As an optical disk is loaded by the user, the DVD player plays back contents data recorded therein by reading out the contents data recorded in the form of optical recording information with the use of a laser beam.
Illegal copying of contents data played back in this manner has becoming a problem in recent years. A technique for preventing illegal copying of contents data has been disclosed, for example, in Patent Document 1. According to Patent Document 1, an encryption key is generated according to an identification text set by the user, and the encryption key thus generated is recorded in an optical disk. When information is played back, the encryption key is read out from the optical disk, and the identification text is obtained from the encryption key that has been read out. Meanwhile, the user is requested to input the identification text. Subsequently, the identification text inputted by the user and the identification text obtained from the encryption key are compared with each other, and when they coincide with each other, of the main information recorded in the optical disk, information specified by the encryption key is played back. In this case, the main information (contents data) recorded in the optical disk will not be played back unless the identification text is inputted. It is thus possible to prevent illegal copying of the contents data.
In the case of Patent Document 1 described above, however, the identification text needs to be inputted each time the content data is played back, and it is, quite a bothersome work for the user to input the identification text. In addition, although it is not easy to manage the identification text, should the user forget the identification text, the contents data can no longer be played back.    Patent Document 1: JP-A-10-283271